This invention relates to a process for producing .epsilon.-caprolactam and more particularly, to a process for producing .epsilon.-caprolactam from cyclohexanone oxime wherein a crystalline zeolite catalyst subjected to a surface treatment with a specific chemical agent is used.
.epsilon.-caprolactam is an important raw material for nylon and the like. One of the processes for preparing the caprolactam is liquid phase rearrangement of cyclohexanone oxime using sulfuric acid as a catalyst.
Furthermore, there have been proposed various processes of gas-phase rearrangement using a solid acid as a catalyst. For example, there has been proposed use of boric acid catalyst (Japanese Published Unexamined Patent Application Nos. 37686/78 and 12125/71), silica alumina catalysts (British Pat. No. 881927), solid phosphoric acid catalysts, mixed metal oxide catalysts (Journal of the Chemical Society of Japan, No. 1, 77, 1977), zeolite catalysts (Journal of Catalysis, 6, 247, 1966) and crystalline alumino-silicate catalysts (Japanese Published Unexamined Patent Application No. 139062/82) or the like.
The above process of using sulfuric acid has problems such as use of a large amount of fuming sulfuric acid, by-production of a large amount of ammonium sulfate, corrosion of apparatuses with fuming sulfuric acid, etc.
For solving these problems, it has been proposed to use various solid acids as described above. However, these processes still have problems in reaction selectivity of .epsilon.-caprolactam, life of catalysts, productivity per catalyst, etc.
For example, Japanese Published Unexamined Application No. 139062/82 shows a specific example where a crystalline zeolite such as ZSM-5 having an atomic ratio Si/Al of 40-60 was used as a catalyst and states that the conversion rate of cyclohexanone oxime is quantitative, but in this case, weight hourly space velocity (referred to as "WHSV" hereinafter) was very low of about 2.5 hr.sup.-1 and life of catalyst was very short, namely, 15-20 hours.
The present inventors have also studied use of ZSM zeolites having the Si/Al atomic ratio as mentioned in the above patent application and have found that not only the life of catalyst, but the selectivity of .epsilon.-caprolactam are not enough and especially under practical WHSV, for example, about 10 hr.sup.-1 or higher, life of catalyst is extremely short and besides selectivity is markedly low.
Thus, use of solid acid catalysts does not simultaneously satisfy the conversion rate of oxime, selectivity of lactam and life of catalysts and further does not provide enough productivity. Therefore, further improvements have been demanded.
Under the circumstances, in an attempt to find a further improved process for production of .epsilon.-caprolactam, the present inventors have made intensive research on crystalline zeolite catalysts having a constraint index of 1-12 and as a result it has been found that when crystalline zeolite catalysts subjected to a surface treatment with a specific chemical agent, not only conversion rate of oxime and selectivity of lactam are markedly improved, but also life of catalyst is sharply prolonged and besides productivity is also improved.